That firmware package is missing the file. A workaround from the bug report:

I'm running buster with 4.16 kernel and firmware-realtek is installed.> It seems that this file should be in the package but it hasn't been> included for some reason, even though it is available from https://github.com/lwfinger/rtl8723au_bt/.>> Installing the file on /lib/firmware/rtl_bt/ solves the issue.

stevepusser wrote:That firmware package is missing the file. A workaround from the bug report:

I'm running buster with 4.16 kernel and firmware-realtek is installed.> It seems that this file should be in the package but it hasn't been> included for some reason, even though it is available from https://github.com/lwfinger/rtl8723au_bt/.>> Installing the file on /lib/firmware/rtl_bt/ solves the issue.

If I use that repo I will have the rtl8821a_config.bin? The tile is rtl8723au_bt.